The invention relates to a method and apparatus for continuous wet-in-wet processing of a length of material as the material is progressively advanced along its length through separate treatment baths. More particularly, the invention comprises the use of a low liquid volume treatment bath which is continuously regulated with additional dosages of chemicals and other preparations.
Wet-in-wet processing should be understood to mean a continuous process in which a length of material, for example a length of textile material, in cut form, in a hank or as a tube, passes through at least two baths or impregnating boxes.
In order to achieve a defined quantity of chemicals, softeners, dye liquors etc. to be applied, attempts have been made to obtain the most accurate difference possible in the moisture content in the length of material using squeezing apparatus or suction arrangements. For this purpose the length of material has as much water as possible removed from it after a first wet processing stage. After the water removal the length of material runs through a dipping tank, an impregnating box or other wetting arrangement, is wetted or soaked with the appropriate liquor and then again brought (by squeezing apparatus or by suction) to a quite specific residual moisture content which in this case must be higher than the moisture content on entering the wetting arrangement. The inlet moisture content can for example be 100% and the outlet moisture content 130%.
The differential moisture content (which in the given example is therefore 30%) must be introduced into the treatment bath by additional dosaging, and all the chemicals and preparations necessary for the desired treatment process must also be contained in this additional dosaging.
In the past there were essentially two methods for the additional dosaging.
The first method uses computers or manual arrangements which determine the weight of the material and the speed of the machine and regulate the additional dosaging accordingly. This method is very costly and difficult to control because of the continuously changing material widths and the differences in the composition and the weight of the particular type of material.
The second method operates on the "carrying off" principle. In this way the liquor level is kept as nearly constant as possible and the additional dosaging is controlled accordingly. However, with this method there are two opposing requirements which are difficult to reconcile.
On the one hand, the smallest possible quantity of liquor is desired (i.e. a small box content) in order to balance the intake and discharge rapidly. The incoming material brings with it for example 100% water (residual moisture) and undissolved preparations which to some extent dissolve in the bath and are mixed together; however, at the same time in this hypothetical example 130% is discharged. The 30% differential moisture content is added by means of a level control and contains all the chemicals and preparations necessary for this process. It will be clear that in this case a small quantity of liquor, i.e. a small box content, is a great advantage since a balance is achieved after a short time (since the outgoing material also carries off dissolved dirt and water).
On the other hand, the additional dosaging has in the past taken place intermittently, being switched on when the theoretical level is reached and switched on again when the level falls below the theoretical level. This results in more or less great surges in level and fluctuations in concentration, since in the interval in which liquor is not dosaged the bath is diluted with introduced water and at the next moment, when dosaging again occurs, increases in concentration. An even treatment, for example, impregnation, of the length of material is not possible in this way.
If by contrast a higher bath content is used, this results in an advantageous buffer effect and the aforementioned fluctuations in concentration do not occur to a marked degree. However, in this case the differences in level are difficult to detect and the dosaging surges take place at greater intervals. This results in the additional disadvantage that the state of balance is achieved considerably later and a larger quantity of waste liquor must be expected in the event of process alterations.